The present invention relates generally to separating sludges and more particularly to the use of insoluble mineral colloidal suspensions that can be added to or formed in situ in sludges and/or floats to aid in separation.
The wastewater produced from different kinds of industries normally contains very fine suspended solids, dissolved solids, inorganic and organic particles, metals and other impurities. Due to very small size of the particles and presence of surface charge, the task to bring these particles closer to make heavier mass for settling and filtration becomes challenging.
Petroleum refining generates large volumes of oily wastewater. With industrial development, there is increase in the amount of oil used. Oil refining, oil storage, transportation and petrochemical industries in the production process generate a lot of oily wastewater.
Separating sludge increases recycled content, reduces the overall volume of waste generated as well as increases the outlets available for disposal. Industrial wastewater sludges are often difficult to separate due to relative density of the components as well as particle size and charge distribution. For example, refinery sludge is becoming increasingly difficult to process as the inorganic content of the crude slate increases. Various processes throughout refineries increase both the organic content in the form of polymer flocculants and the inorganic content captured by the flocculants.
The low density of particulates in the sludge combined with the low density of the accumulated spent flocculants and the increased density of the water component, with high dissolved solids, results in poor separation.
Various traditional and advanced technologies have been utilized to remove the colloidal particles from wastewater; such as ion exchange, membrane filtration, precipitation, flotation, solvent extraction, adsorption, coagulation, flocculation, biological and electrolytic methods.
Traditional approaches include the addition of bentonite, lime, ferric compounds, or varying combinations of expensive coagulants and flocculants. These are often ineffective or require significant dosing rates.
Petroleum refiners use Dissolved Nitrogen Floatation (DNF) units for clarifying wastewater from a variety of refining processes that include washing from a desalter and other wastewater generated processes within a refinery.
DNF units force nitrogen under pressure in the form of microscopic bubbles often with a coagulant additive to float particulate matter that has densities near that of water to the top of the unit where they are skimmed off. Solids that are higher in density sink to the bottom where they are collected separately. The middle phase which is absent of solids is sent to wastewater treatment for processing before discharge.
The DNF bottom sludge and top float are often combined for further treatment. According to one aspect the present invention relates to processing of the combined DNF sludge and float.
Coagulation is the destabilization of colloidal particles brought about by the addition of a chemical reagent known as a coagulant. Flocculation is the agglomeration of destabilized particles into microfloc, and later into bulky floccules which can be settled called floc.
Coagulation is the process by which colloidal particles and very fine solid suspensions initially present in a wastewater stream are combined into larger agglomerates that can be separated by means of sedimentation, flocculation, filtration, centrifugation, or other separatory methods. This involves a chemical process in which destabilization of non-settleable particles is realized. These non-settleable particles can include most colloids as well as extremely small solid particles and all solvated (dissolved) particles. These particles form clumps with the help of a coagulant. Coagulation is commonly achieved by adding different types of chemicals (coagulants) to the wastewater stream to promote destabilization of any colloid dispersion present and the agglomeration of the individual resultant colloidal particles. Coagulation is the destabilization of these colloids by neutralizing the electrostatic forces that keep them apart. Cationic coagulants provide positive electrostatic charges to reduce the negative electrostatic charges (zeta potential) of the colloids. As a result, these particles collide to form these larger floc particles.
Flocculation refers to the coming together of particles by means of a physical or mechanical process resulting in the joining together of large aggregated clumps (or flocs) to form larger masses and eventually to precipitate them from the liquid phase and thereby convert them into the solid phase for further separation. In coagulation, these forces responsible for keeping the particles suspended and dispersed after they contact each other are reduced. This is usually referred to as collapsing the colloid and/or precipitate formation in the case of solvated particles. Flocculation joins these de-established colloidal dispersions into large aggregates that enter the solid phase.
According to the present invention the addition or in situ formation of insoluble mineral colloidal suspensions in DNF sludges and floats is used to destabilize DNF sludge and/or float wastes for purposes of separation and component recovery. Further, these materials have improved separation in DNF, Dissolved Air Flotation (DAF), and American Petroleum Institute Separator (API Separator), and tank clean-out sludges.